1. Field of the Invention
The field of the invention is high performance, electronic, motor drives for variable speed control of AC induction motors, and more particularly, motor drives using vector control techniques and speed feedback.
2. Description of the Background Art
Vector control or field-oriented control is one technique used in motor drives for controlling the speed and torque of AC motors. With this technique, stator current is resolved into a torque-producing or q-axis component of current, I.sub.q, and a flux-producing or d-axis component of current, I.sub.d, where the q-axis leads the d-axis by 90.degree. in phase angle.
To provide a high performance drive, there are several other requirements. A speed sensor is required, to obtain speed feedback from the rotor, which is used in controlling the torque, frequency and slip at which the motor is operated. Another requirement of prior drives has been a knowledge of motor parameters such as inductance (L) and resistance (R) of the rotor and stator. In prior systems, the set up of a drive involved adjustments based on these parameters for the particular motor being controlled.
While other motor control techniques are known to reduce the number of motor parameters which must be evaluated, they have not altogether eliminated this requirement in a high performance drive. The motor parameters must be obtained from the manufacturer of the motor or determined through rigorous testing of the motor.
Besides making an initial determination for slip, based on motor parameters, another requirement has been on-line adaptation to dynamic changes in motor parameters during operation of the motor. One example of a dynamic change occurs when the rotor resistance (R.sub.r) changes with the heating of the motor.
Additional control strategies are required when operating in the constant horsepower region, above base speed, where it is necessary to (1) weaken flux to achieve higher speeds and (2) maintain the vector control relationship of the d-axis and q-axis components of flux produced in the motor.